Nmap ("Network Mapper") is a free open source utility for network exploration or security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics.

NmapFE is definitely a great way to go. Unfortunately, developement has stopped and it was last compiled with nmap 3.70, which is a bit dated and missing out on some of Nmaps new features (better OS and service detection).